Use PHALCON_THROW_EXCEPTION_STR instead of phalcon_throw_exception_string

  * phalcon_throw_exception_XXX() functions do not restore the memory frame anymore (and thus can be safely used from the kernel);
  * PHALCON_THROW_EXCEPTION_STR and PHALCON_THROW_EXCEPTION_ZVAL macros take care of restoring memory frame
  * PHALCON_THROW_EXCEPTION_STRW and PHALCON_THROW_EXCEPTION_ZVALW are just aliases for phalcon_throw_exception_string and phalcon_throw_exception_zval
  * all calls to phalcon_throw_exception_string replaced with PHALCON_THROW_EXCEPTION_STR macro